ABSTRACT: The long-term goal of this grant proposal is to understand at an atomic level how the ribosome functions during protein synthesis. We will take a multidisciplinary approach utilizing genetic, biochemical and structural methods to study the bacterial ribosome. Our first aim is to examine ribosome structural dynamics during decoding of mRNA and its inhibition by antibiotics. We will use X-ray crystallography to obtain high-resolution structures of mutant Thermus thermophilus 30S subunits that reveal new conformational states. We will examine the effects of the mutations on the accuracy and kinetics of decoding. Our second aim is to probe a pathway by which codon recognition is signaled via ribosomal protein S12 and aminoacyl-tRNA to EF-Tu to activate GTP hydrolysis, a key step in the tRNA selection process. These experiments are designed to provide fundamental insights into ribosome structure and function, including new conformational states of the ribosome, to aid in understanding the decoding process. The ability to solve structures of ribosomes with antibiotic-resistance mutations will aid in understanding the mechanisms of antibiotic action and resistance, and has particular clinical relevance for the rational design of drugs to fight resistant microbial pathogens.